Guarantees (Details 3)	Sep. 30, 2015 USD ($)
Guarantor Obligations [Line Items]
Amount of hedging obligations guaranteed reflected on balance sheet	$ 0
Guarantor obligations, maximum exposure, undiscounted	125,900,000
Fixed maximum amounts guaranteed by year 2015	2,800,000
Fixed maximum amounts guaranteed by year 2016	26,400,000
Fixed maximum amounts guaranteed by year 2017	22,900,000
Fixed maximum amounts guaranteed by year 2018	500,000
Fixed maximum amounts guaranteed by year 2019	56,800,000
Fixed maximum amounts guaranteed by year expires after certain number of days after written notice	12,500,000
No scheduled maturity date	4,000,000
Amount outstanding under guarantees that is reflected on balance sheet	200,000
Letters of credit	51,900,000
Letters of credit set to expire - 2015	12,900,000
Letters of credit set to expire - 2016	34,800,000
Letters of credit - no scheduled maturity date	4,200,000
Outstanding letters of credit	1,500,000
Natural gas transportation and storage agreement fixed maximum amount of performance guarantee of a related party	4,000,000
Amount outstanding by a related party under the guarantee	1,100,000
Amount of surety bonds outstanding	$ 538,100,000